Vanishing Edges 101

Remember the first
time you saw a
vanishing-edge pool? Once you got
past the sheer beauty of the effect,
you probably wondered, "How'd they
do that?"

In the 21st century, vanishing edges
— also called negative edges or infinity
edges — are common enough that
lots of the engineering puzzles have
been worked out. But it's still a detail
that you probably shouldn't attempt
on the first pool you build.

Mike Sorenson, manager of the San
Gabriel, Calif., office of California
Pools has 30-plus years of experience
in pool building, and he graciously
shared the wisdom he's accumulated
on the basics of vanishing edge
details. Sorenson talked about four
topics: surge-tank capacity and
design, the over.ow edge, service and
maintenance issues and the over.ow
wall. Of course there are more than
four areas to consider when designing an infinity edge pool, but with
these recommendations in mind,
you'll be on your way.

LESSONS LEARNED

Sorenson says understanding the
concept of surge capacity is key. "In a
typical pool if a bunch of people jump
in it, all you get is waves, and a few of
those waves will lap out over the deck
and onto the lawn. With an infinity
pool, because there is no edge on the
far side, if 15 kids at a birthday part
jump into the pool, they're going to
displace 8,000 to 10,000 gallons of
water, and that water has to be captured and returned to the pool.

"That capturing process happens in
the surge chamber. The surge chamber will always have some amount of
water in it — ambient water." Sorenson says the amount of ambient
water (and therefore the size and configuration of the tank) is determined
by how the homeowners intend to
use the pool.

"When people start swimming in
the pool, pumps are either manually
turned on, or if there are electronic
.oats in the surge chamber that sense
when water is filling inside of it, the
pumps will automatically turn on and
start pumping water back into the
pool. If you mostly use your pool to
lounge about in the water, the
amount of surge you'll create is
nowhere near a family of six that has
the soccer team over on Saturday.

"So a family that swims less aggressively will tend to keep the ambient
water level higher in the surge chamber with less surge capacity — the
vacant space that the surge chamber
has available to catch water while the
pumps are trying to catch up. So really aggressive swimmers will keep the
ambient water level a little lower and
allow more vacant space, so that as
they start swimming, the pumps will
have more room, more .exibility.

"You might ask, 'Why don't you just
keep the surge chamber empty.' We
want to cycle that water in with the
pool water to keep the chemistry balanced. If the surge chamber is dry or
nearly dry, the pumps can pump it
empty and lose prime. So you always
want to have some amount of water
in the surge chamber."

Sorenson says learning to size the
surge chamber correctly was a skill
learned through trial and error. "We
didn't understand that in the '80s
until we had a number of these out in
the field where people were swimming, and we were either running
our pumps dry because [the surge
chambers] were too small, or we were
flooding them because there wasn't
enough storage capacity." Surge-chamber size, configuration and placement are at the heart of a vanishing-edge pool.

While there are some basic formulas for sizing the tank, it's still up to
the pool builder to make allowances
for each individual project. "The commercial world deals with these pools
in a different way, and they're the
originators of the formulas. When
you swim in a giant pool at the
YMCA, for example, it has a scum
gutter — an infinity edge — around
the perimeter. Their formula is one
gallon of surge capacity per
square foot of pool."

It's important to consider the
likely number of swimmers in
relation to the pool's surface
area. "A large pool can absorb
the turbulence better," says
Sorenson. "As people are doing
cannonballs, by the time the
wave reaches the edge of a large
pool, it's just a ripple. In a
smaller pool with the same
number of people, you're going
to have more surge.

"A commercial pool is going to
be a huge 2,000- or 3,000square-foot pool, and typically they
plan for one swimmer per 15 square
feet. The ratio of swimmers in a
home pool is much higher. So that
one gallon per square foot formula
that the commercial world uses is not
enough in a residence. A new builder
— someone who is planning his first
infinity edge pool — will want to take
that formula and add 400 gallons to it."

TANK SHAPE

Sorenson says that figuring the correct capacity for the tank isn't
enough. Its placement in relation to
the infinity edge and its configuration
are important, too.

When the water spills over the
knife-edge, not only does it fall down,
but it travels out along a trajectory. Simply put, the farther below the
edge the surge chamber is located,
the wider the surge chamber will
need to be, in order to catch water
that has traveled horizontally.

"If big waves are going over that
wall, some of the water may travel far
enough horizontally over the side that
it doesn't catch in the surge chamber. There's no hard-and-set formula; it's
an art. They are never the same,
because every pool, every location,
every homeowner is different."

THE EDGY LOOK

"The main effect of an infinity pool is
that rim-.ow look; that serene, water
disappearing off the face of the earth. I call it the Christopher Columbus
effect. Part of the success is to hide or
disguise the infinity edge. When we
first built these, the point where the
water flows over the edge was horizontal and the water traveled the
width of the wall and then spilled
over.

"We began beveling that edge away
from the angle of view, to create an
actual point. So the plane of that
knife-edge is facing away from the
viewer and is less apparent; there's
just a point where the water pours
over the edge.

"The location of the pool of course
is important, where it's seen from the
house. Consider where the windows
and doors are, the seating areas and
patios. The location of the pool plays
into all of that.

"Another element of the illusion is
the distance from the top of the infinity wall to the top of the surge chamber. You'll need to consider the
height so the top of the surge chamber is not seen from the opposite side
of the pool."

INFINITY SERVICE

Infinity edge pools require a little different approach to service and maintenance. Evaporation and the way the
pool refills are two factors that set
them apart from ordinary pools.

"When the service guy comes to the
house to clean the pool, he always
looks at the water level and if the
pool's a little low he'll add water. Infinity-edge pools are always completely full, because the pumps fill the
pool up to the level of the edge. So the
service guy visits that pool every week
and it always looks full to him, but he
doesn't realize that the water level
that has to be maintained is in the
surge chamber, not the pool."

And it's important to remember that
adding six inches to the surge chamber does not equate to adding six
inches to the pool. "Think about
how long it might take to fill a
coffee cup with a thimble"
Sorenson says. "You could pour
the entire thimble into the coffee cup and you will have depleted the thimble and barely made
a difference in the coffee cup. If
you want to add an inch of
water to the pool, it will typically
take 15 to 20 inches out of the
surge chamber.

"Let's say we had a big party
last night and then shut the
pool off as we were getting out,
and the surge chamber is full to
capacity and the water in the pool is
down below the tile border. Maybe we
even allowed the surge chamber to
over.ow and actually lost water from
the system. So let's say the pool water
level is two inches low, and the surge
chamber is full. When the pumps
turn on and try to replenish those two
inches, they are going to pump the
surge chamber completely dry. Because there's simply not enough
water in the surge chamber to refill
the pool.

"So we put an automatic filling
device [which adds water from outside the system] in the surge chamber
so it will never go below a certain
point. We set that based on how we
interpret the way the pool will be
used.

"In the early days we weren't prepared for how much evaporation
occurs when water flows over the
infinity wall for long periods of time. We had many customers who
thought that their pools were leaking.

"With an auto fill in the surge
chamber, that will never be noticed. But some contractors don't like auto
fills with the inherent risk that comes
with them — something could break
or the auto fill could keep running
and .ood something, or if there is a
leak, no one knows because the auto
fill is keeping the pool level. So some
people don't like those.

"So if they're manually adding the
water into the surge chamber they are
going to be doing that a couple of
times a week."

BEHIND THE CURTAIN

While builders often think long and
hard about the interior surface of the
pool walls — Tile. Pebble. Some
other coating. — they rarely need to
worry about the outside surface. It's
usually not visible. Not so with an
infinity edge. The over.ow wall usually must look good and always must
wear well.

"There has to be some material on
the outside face of the pool wall that
is below the knife edge that can survive with water on it or out in the
sun," says Sorenson. "That material
will be based on the customer's taste
and budget. Often we will carry the
tile from the knife-edge down the face
into the surge chamber at some level. That's the most expensive approach,
but it looks the nicest.

"Occasionally we'll build one of
these where 99 percent of the view is
from the house and no one but the
coyotes will ever see the downhill
side. So some customers will use a
watertight product like Thoroseal that
looks like gray paint, and it will seal
the wall. It's not that pretty, but it
saves a lot of money."

KEEP IT CLEAN

After engineering the perfect knifeedge and the correct plumbing and
circulation to make the vanishingedge effect work, Sorenson discovered one more maintenance rule. "We learned that it's not a good idea
to .ow water over that knife edge for
hours and hours at a time." One reason is the evaporation problem discussed above. The other reason is
maintenance. "Over time as water
flows everyday, you start to get a
buildup of calcium," Sorenson says. It's especially problematic if the
homeowner has a patio nearby or
some other view of the wall.

"In our pools now, we don't let that
water .ow for hours and hours a day. We have pumps that are dedicated to
moving water from the surge chamber to the pool. That pump is turned
on when people are swimming, and
even if people are not swimming,
we're going to run the water from the
surge chamber into the pool so it
doesn't get stagnant. The edge is overflowing for only about 30 minutes
each day, and that's going to reduce
the calcium buildup on that outside
wall.

"As long as the pool is entirely full,
it doesn't look any different from the
house whether the water is running
or not," says Sorenson. "So it doesn't
make the customer any happier to
have it running constantly."

Kirstin Pires, has been on the editorial staff of AQUA magazine since 2002. She has a B.A. in art from Carleton College and M.A. in journalism from the University of North Carolina at Chapel Hill, N.C., and currently lives in Madison, Wisc.

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